1. Field of the Invention
The invention relates to a bushing for gas-insulated metal-clad electrical switchgear comprising a substantially cylindrical metal jacket surrounding a conductive busbar.
2. Description of the Prior Art
The invention relates more particularly to a bushing intended to be mounted on the tank of metal-clad high-voltage or medium-voltage switchgear such as a circuit-breaker. The person skilled in the art conventionally refers to the mounting of a bushing on a tank as an xe2x80x9cofftakexe2x80x9d, this term also designating the opening provided in the tank to receive the bushing. This type of tank, usually called a dead tank, is electrically grounded. In the above kind of installation, the bushing can be disposed vertically relative to the tank if the latter is horizontal and is fixed to the ground, but can also be disposed in other directions. The bushing includes a conductive busbar that provides the connection between a conductor outside the jacket and a conductor or electrical switchgear inside the jacket and surrounded by a dielectric gas such as SF6. A portion of the conductive busbar is outside the tank and is electrically isolated from the surrounding air by a sleeve or by a molded electrically insulative material that covers as far as its end a particular length of the busbar, which is conventionally connected to a high-voltage or medium-voltage conductor. This type of bushing generally has a substantially cylindrical metal jacket surrounding the conductive busbar and connected to an opening formed in the tank. The jacket is welded to a flange of the tank, for example, to form with the tank a gas-tight combination containing the insulative gas. The conductive busbar enters the tank via this opening and is electrically connected to a live component of the electrical switchgear. One end of the metal jacket is conventionally fixed to the external perimeter of the lower end of the sleeve. In the following description, the term xe2x80x9csleevexe2x80x9d is used interchangeably to designate an actual removable sleeve or a molded material, in which case removal of the insulative member is not possible.
If an electrical arc is struck between the circuit-breaker. tank and the conductive busbar, the electromagnetic forces cause the arc to move along the conductive busbar to the bushing, where it stabilizes between the conductive busbar and the metal jacket and against the end surface of the insulative sleeve that is in contact with the gas contained in the jacket. In the following description, the expression xe2x80x9cinside face of the sleevexe2x80x9d refers to this end surface. The arc carries a very high current that can melt the metal jacket. The arc typically perforates the metal jacket and can even cut through it by rotating about the conductive busbar. This phenomenon causes molten metal and hot gases to spray out of the metal jacket. This kind of accident can occur even if a relief valve is provided to evacuate the pressurized hot gases to the exterior of the tank in a controlled direction. This kind of accident can be hazardous to any operators in the vicinity.
To improve the resistance to electrical arcing, the thickness of the metal jacket is conventionally increased, which significantly increases the cost of this kind of bushing, in particular because a stainless metal is generally employed for the jacket.
Another conventional solution to improving resistance to arcing consists of disposing an electrical member at a particular location in the tank of the electrical switchgear, connected to the tank and adapted to be sacrificed by the arc. A reduction of the dielectric distance between a phase and ground at this point favors the striking of the arc at this point in the event of an abnormal overvoltage, the arc stabilizing between this member and a live component of the switchgear, with the result that it eventually melts the member without damaging the tank. However, the electromagnetic forces attracting the arc along the conductive busbar mean that it is not possible to guarantee that the arc will stabilize on the sacrificial member. If the arc is not stabilized on the sacrificial member, or is struck elsewhere than on the sacrificial member, it travels along the bushing and pierces the metal jacket, with the consequences previously mentioned.
The object of the invention is to remedy this drawback by proposing a bushing that is capable of avoiding spraying of molten metal or hot gases out of the bushing in the event of internal arcing.
To this end, the invention provides a bushing for gas-insulated metal-clad electrical switchgear accommodated inside a tank, including a conductive busbar extending along an axis and surrounded over a portion of its length by an insulative sleeve, and a substantially cylindrical metal jacket which contains the gas and surrounds another portion of the length of the busbar, one end of the jacket being open so that it can be connected and sealed to the tank, and another end of the jacket being closed by the sleeve, which has an inside face constituting its end surface in contact with the gas contained in the jacket, which bushing includes a conductive material annular member surrounding the jacket and situated at substantially the same level as the inside face of the sleeve along the axis of the conductive busbar (4) and includes a cap surrounding the jacket and adapted to form with the jacket a closed space delimited by the annular member at one end.
With the above construction, when the arc travels along the bushing, it is blocked by the inside face of the insulative sleeve and consequently pierces the metal jacket before attaching itself to the annular member. Any molten metal from the annular member sprayed out of the metal jacket is contained within the closed space.
In a preferred embodiment, the annular member comprises two half-flanges which are screwed together so that they can be clamped onto the metal jacket and the cap is circular and is fixed by force fitting it, which reduces the cost of fabrication of the bushing.
In another preferred embodiment, the outside surface of the annular member is entirely covered with an insulative material sheath to prevent any molten metal from spraying out in the event of very high current electrical arcing.
The invention is described in more detail hereinafter with reference to the accompanying drawing, which shows one embodiment of the invention by way of nonlimiting example.